This invention relates generally to refrigeration cycle efficiency, and more particularly to recovery of energy in a process where a gas is expanded from a supercritical region into a wet region of the cycle.
The need for refrigeration systems using environmentally benign refrigerants has become greater in the last decade because of the dual drivers of ozone depletion and global warming. xe2x80x9cOptimalxe2x80x9d refrigerants that were engineered to maximize the cycle efficiency by minimizing the expansion throttling loss are no longer usable. The major air conditioning manufacturers have adopted refrigerants which either have a high vapor pressure and hence throttle loss (R-134A), or which have partial ozone depletion potential and high toxicity level (40 ppm Allowable Exposure Limit) and are due to be phased out (HFC-123).
The choice of CO2 as a refrigerant has many benefits. It is a natural, non-toxic substance with no ozone depletion potential. Additionally, there are volumetric and heat transfer advantages and the refrigerant has a low cost. A disadvantage is that the high pressure difference for compression and expansion result in a high throttling loss, leading to a poor cycle efficiency compared to current HCFC refrigerants.
However, a supercritical CO2 refrigeration cycle with an expander was found to have the same or better performance as an HCFC cycle with expansion valve. The gain in CO2 refrigerant cycle efficiency resulting from energy recovery with a 60% efficient expander was as much as 33% compared to a CO2 cycle with a throttling valve, and 25% compared to the same throttling valve cycle with maximum internal heat exchange.
The need for an expander to recover energy from the expansion is clearly indicated if the CO2 refrigeration cycle is to be widely deployed. However, the expansion, which starts in the supercritical region, enters the two-phase region, producing over 50% liquid. by mass.
No practical expanders have been developed for this range of operation from the supercritical region into the saturation region (transcritical expansion). Attempts to use radial inflow machines in the wet region have not been successful due to poor performance and erosion from the liquid centrifuging outward. Attempts to use positive displacement machines have not been, successful due to high cost and size and reliability issues.
Another requirement for expanders, to improve the efficiency of a refrigeration system, is a cost effective method to use the generated shaft power.
A primary objective of the invention is to provide an efficient, cost effective method of recovering energy in a process where a gas is expanded from the supercritical region into the wet region of the cycle. Another important objective is to provide an efficient, cost effective means and method to utilize the power generated by the above expansion to reduce the power required by the process, for example by a compressor in a refrigeration cycle.
Another object is to provide a method of operating an axial flow turbine having rotor blades rotating about an axis, employing a working fluid capable of two-phase flow, that includes:
a) vaporizing said fluid
b) compressing the vaporized fluid to a supercritical state,
c) cooling the compressed vaporized fluid while maintaining it in said supercritical state,
d) providing and operating a nozzle to receive and expand the compressed vaporized fluid to a pressure and temperature in the wet region of the cycle characterized by formation of liquid phase fluid droplets in the two-phase flow from the nozzle,
e) directing that two-phase flow toward the turbine blades whereby the flow is turned in the spaces between the blades and directed to flow axially of the turbine rotor as well as outwardly away from its axis, producing a swirl of the flow leaving the blades, which typically reduces, and operable to produce torque transferable to the rotor, and to act in the direction of rotor rotation.
f) and subsequently vaporizing said flow that leaves the blades pursuant to step a).
A further objective is to provide a turbine that comprises:
a) an expansion nozzle contoured to expand the high pressure, supercritical fluid to a lower pressure in the wet region, producing a high velocity directed flow of gas and sub-micron liquid droplets or of supersaturated gas,
b) axial flow blades, attached to a rotor, directed to receive the high velocity flow and turn it, generating torque acting on the rotor,
c) surfaces oriented to receive liquid from the turned flow and direct it away from the moving (rotating) rotor,
d) an exit duct to remove the gas and liquid flow from the flow directing surfaces and moving rotor,
e) and a shaft coupled to the rotor to transfer generated torque to a load.
An additional objective is to provide an assembly consisting of the above turbine elements and a load that includes a compressor to increase the pressure of a gas whereby power for the compressor is provided at least in part by the power generated by the turbine.
Yet another objective is to use the above assembly to increase the pressure of the flow leaving the turbine, after the liquid is evaporated in a refrigeration system or heat pump, thereby reducing the power required by the main compressor for the system.